1. Field of the Invention
This invention relates to digital communications systems and, more particularly, to a switching unit for routing data through a time-division digital communications system.
2. Description of the Prior Art
It is often desirable to exchange digital messages between digital devices. U.S. Pat. No. 3,749,845 issued to A. G. Fraser on July 31, 1973 discloses a digital data communications system including a network of interconnected switching units, each switching unit having connected thereto a transmission loop and each loop having one or more digital devices attached thereto. The prior art system provides controllable buffering of digital data, thereby allowing digital devices having different data transfer speeds and storage capabilities to communicate asynchronously. The system creates an actual communications path only when the requesting device is transmitting data. Thus, other system resources need not remain committed between bursts of data.
The data to be transmitted between the digital devices is typically formatted into a plurality of fixed-length segments called packets. Each packet includes header routing information for identifying a destination device. The header, usually unique at least on a communication channel between two switching units, is used by intermediate switching units for routing the packet through the network toward its destination.
A central component of the known switching unit is a digital computer operating as a packet switch. The computer is programmed to allow dynamic prescheduling of requested communications paths, thereby mitigating the total communications time and cost.
The prior art switching unit, called a demand-sharing unit, obtains economies by taking advantage of the usually bursty nature of the data. However, a demand-sharing unit typically employs more complex, and hence more expensive, switching equipment than would a switching unit incorporating the time-division art. As the number of devices served by a communications channel increases, the resultant transmitted data will typically approach a steady level, which is economically consistent with the time-division art. Accordingly, it is an object of the invention to provide a less expensive packet switching unit for routing data through a time-division digital data communications system.
Unfortunately, known packet switches have rarely obtained high throughput, i.e., throughput in excess of one million bits per second. This problem stems partly from packet routing overhead concomitant to the packet switch digital computer employing many processor cycles in routing a packet. Throughput can be increased by increasing processor speed; however, increasing the processor speed typically has an ensuing increase in packet switch cost. On the other hand, a packet switch operating in a digital communications system having transmitted data concentrated onto one or a few channels, for example, by time-division multiplexing techniques, can achieve high throughput. Accordingly, it is a further object of the invention to provide a less expensive packet switch having improved throughput consistent with packet routing overhead.
Concentrating data onto a few channels often leads to a contention problem. The contention problem arises from independent devices transmitting packets which may arrive simultaneously at a point. The contention problem can be overcome by queuing; however, queue control then becomes a problem. It is a still further object of the invention to provide a less expensive packet switch with an improved queue-control arrangement.
Known packet switches are not of modular construction. Hence, growth of a digital data communications system, as, for example, growth which arises by adding more digital devices to the system, has been economically stymied. Accordingly, it is still another object of the invention to provide a modular packet switch.